1. Field of the Invention
The present invention relates to a transmission line, a filter, a duplexer, each being for use in a microwave band, and a communication device including them.
2. Description of the Related Art
It has been known that by periodically changing the line impedance of a transmission line in the transmission direction of a signal, a frequency characteristic intrinsic to the transmission line can be presented, as descried in Vesna Radisic etc, xe2x80x9cNovel 2-D Photonic Bandgap Structure for Microstrip Linesxe2x80x9d, IEEE MICROWAVE AND GUIDED WAVE LETTERS, Vol. 8, No. 2, FEBRUARY 1998 (Literature 1), Fei-Ran Yang etc, xe2x80x9cA Novel Compact Microstrip Bandpass Filter with Intrinsic Spurious Suppressionxe2x80x9d, Asia-Pacific Microwave Conference Digest December 1998 (Literature 2). The Literatures 1 and 2 show that electrode-removed portions are arranged in the earth surface of a microstrip line at equal periods in the signal propagation direction and in the perpendicular direction to the signal propagation direction.
However, in the case of designing a filter by use of such a transmission line of which the impedance is periodically changed, it is difficult to design a filter having a predetermined filter-characteristic by connecting the transmission lines to each other, since the shape of the signal propagation line portion becomes complicated.
A low-pass characteristic can be rendered to a transmission line such as a microstrip line by forming an electrode-removed pattern in the earth surface thereof. However, the literatures 1 and 2 describe that the electrode-removed patterns are arranged at equal intervals in the signal-propagation direction and in the perpendicular direction thereto. Accordingly, the frequency of the stop-band can not be optionally determined. For example, if the intervals between the above-described electrode-removed patterns are changed in order to change the frequency of the stop-band, the characteristic impedance of the transmission line is changed, and the reflection characteristic is deteriorated, problematically causing the transmission loss to increase.
To overcome the above described problems, that is, the deterioration of the reflection characteristic and the increase of the transmission loss, preferred embodiments of the present invention provide a transmission line, a filter, a duplexer, each having a desired frequency characteristic, and a communication device including them.
One preferred embodiment of the present invention provides a transmission line comprising: a signal propagation line portion; and a ground electrode in correspondence to the signal propagation line portion, the ground electrode defining a ground electrode formation surface; wherein the electrode non-formation portions are formed in the ground electrode formation surface so as to be distributed at substantially equal intervals in a signal propagation direction and at intervals in the perpendicular direction to the signal propagation direction, at least one of the intervals in the perpendicular direction being different from the intervals in the signal propagation direction.
According to the above arrangement, the electrode non-formation portions are arranged at substantially equal intervals in the signal propagation direction. Thus, a frequency in correspondence to the intervals and the wavelength on the transmission line can be determined as the center frequency in the stop-band. The impedance of the transmission line and the attenuation in the stop-band can be determined by setting the intervals of the electrode non-formation portions in the perpendicular direction to the signal propagation direction, independently of the intervals in the signal propagation direction.
Preferably, the intervals of the electrode non-formation portions substantially in the perpendicular direction to the signal propagation direction are changed in correspondence to the line impedance of the signal propagation line. For example, the impedance matching is carried out on the way of the transmission line. Reversely, the impedance is changed on the way of the transmission line.
Further, according to the present invention, there is provided a filter which comprises the above-described transmission line. That is, the band-stop characteristic of the transmission line itself is used as a filter-characteristic.
Preferably, in the filter of the present invention, the above-described transmission lines are provided as plural resonance lines, adjacent resonance lines thereof being coupled to each other. Accordingly, the filter has both of the band-stop characteristic caused by the above-described electrode non-formation portions and the frequency characteristic caused by the resonance lines.
Another preferred embodiment of the present invention provides a duplexer which comprises two sets of the above-described filters. For example, the above filters are provided as a transmission filter and a reception filter to constitute an antenna sharing device.
Yet another preferred embodiment of the present invention provides a communication device in which the above-described transmission line, filter or duplexer is used.